1. Field of the Invention
The present invention relates to buoyancy compensation devices for use by underwater divers, especially recreational SCUBA divers, and more particularly to a buoyancy compensation device that provides relatively constant buoyancy regardless of the diver's depth and a method of providing buoyancy compensation at relatively constant buoyancy for an underwater diver regardless of the diver's depth.
2. Description of the Prior Art
Typically, a SCUBA diver with dive gear, including a wetsuit and air tank, is positively buoyant and must wear lead weights in order to submerge. When the diver enters the water, the wetsuit has maximum buoyancy owing to the air in the pores of the expanded neoprene or butyl rubber from which wetsuits are typically made. As the wetsuit gets soaked with exposure to water and the air in the wetsuit is compressed as the diver descends to depths having pressures greater than atmospheric, it loses some of its buoyancy. Thus, the diver may have to overcome the tendency to sink. SCUBA divers typically use a “buoyancy compensation device” (BCD) to adjust buoyancy during a dive.
The BCD in current, relatively widespread use by divers, referred to herein as a “conventional BCD,” is simply a substantially inelastic, inflatable bladder that can be filled and emptied of air that is always at ambient pressure. That is to say, the internal pressure of a typical inflated BCD bladder corresponds substantially to the external ambient pressure regardless of water depth; at the water surface the BCD bladder internal pressure is 14.7 psia and at about 150 feet water depth the BCD bladder internal pressure is about 80 psia, the ambient pressure in fresh water at about 150 feet depth.
By inflating the conventional BCD when at depth, the diver's tendency to sink owing to the compression of the diver's wetsuit, for example, can be overcome. As the diver uses up compressed air from the air tank, the tank becomes lighter, and thus more positively buoyant, typically, by about 5 lbs., which corresponds to about 5 lbs. of lift or about 2.25 liters of air volume. Thus, at the end of a dive, it may be difficult for a diver to stay submerged for a safety stop (to prevent decompression illness). Should the diver experience such difficulty, the diver can carry more lead on the next dive and, to be safe, can overestimate the lead requirement. Thus, according to this approach, the diver will carry a fairly large amount of lead and then compensate for the added weight by introducing air into the BCD bladder. Apart from having to carry the extra weight, this presents another problem for the diver.
The air that is forced into the conventional BCD bladder from the air tank displaces a given volume of water at a given depth, i.e., the deeper the dive, the more air will be required to produce the same volume displacement of water. The lift or buoyancy generated is equal to the weight of the water displaced from the BCD bladder. It would be desirable for this displacement to remain constant throughout the dive, regardless of the diver's depth. However, the air in the BCD bladder is compressed as the diver descends to deeper depths (producing less displacement and less lift) and expands when the diver ascends toward the surface (producing more displacement and more lift). The former situation is a nuisance; as the diver descends, he may find that he has a tendency to sink faster and must add air to the BCD bladder to compensate for the faster descent. However, the latter situation is not only a nuisance, but can be quite dangerous.
A brief description of lung volumes and their definitions will be helpful in understanding the following description of the prior art devices, as well as the invention and its operation and advantages over the prior art devices. A typical representation of lung volumes measured at atmospheric pressure is shown in the chart of FIG. 10 and in the following table:
Lung VolumeVolume in litersDescription/DefinitionTotal lung capacity6.0The volume of air contained in the lungs at the(TLC)IRV + TV + ERV + RVend of maximum inspiration. Total volume of thelungs.Vital capacity4.6The volume of air that can be expelled from the(VC)IRV + TV + ERVlungs after a complete maximum inspiration.Tidal Volume0.5The volume of air breathed in or out of the lungs(TV or VT)during normal respiration.Residual Volume1.2The volume of air left in the lungs after a(RV)maximum exhalation.Functional Residual2.4The volume of air that stays in the lungs duringCapacity (FRC)ERV + RVnormal breathing.Inspiratory Reserve3.0The maximum volume of air that can be inspiredVolume (IRV)VC − (TV + ERV)into the lungs in addition to the tidal volume.Inspiratory Capacity3.5The maximum volume of air that can be inspired(IC)TV + IRVinto the lungs following a normal expiration.Expiratory Reserve1.2The volume of air that can be expelled from theVolume (ERV)lungs after the expiration of a normal breath.
Based on the chart and table, it can be seen that during normal breathing at tidal volume (TV) ±0.5 liter, a diver changes his buoyancy or lift only about ±1 lb. It can also be seen that if a diver inspires deeply after a normal inspiration, he can increase the air volume in his lungs by about an additional 3.0 liters (IRV) and thus increase his buoyancy by about 6.6 additional pounds. It can also be seen that if a diver exhales deeply after a normal expiration, he can decrease the air volume in his lungs by about an additional 1.2 liters (ERV) and thus decrease his buoyancy by about 2.6 pounds. A skilled diver can maintain a normal tidal volume, but increase his ERV and decrease his IRV so that he is breathing “near the top” of his vital capacity. Similarly, a skilled diver can maintain a normal tidal volume, but decrease his ERV and increase his IRV so that he is breathing “near the bottom” of his vital capacity. This permits the seasoned diver to adjust his buoyancy during the course of a dive to a magnitude that can come close to the changes in buoyancy caused by using up air and compressing his wetsuit.
If a diver ascends to the surface using a conventional BCD without carefully controlling his rate of ascent by releasing air from the BCD, the air in the BCD will increase in volume making him ascend faster, which further increases the BCD volume and so on, resulting in an “uncontrolled ascent.” An uncontrolled ascent can cause two serious problems. One of these problems is decompression sickness, commonly called “the bends,” which is caused by the release of dissolved nitrogen as a gas into the blood stream. An equally, if not more, serious problem can be caused by a rapid increase in the total lung volume. A skilled, relaxed diver knows to allow air to expel from the lungs during such a situation. A novice or panicked diver may keep his glottis closed. This, in turn, will cause an increase in lung volume beyond that which the lungs are capable of accommodating, which may rupture the alveoli, resulting in a pneumothorax. In such a situation the lung collapses and the air escapes into the chest. A pneumothorax can be fatal and is a common cause of death for the novice or panicked diver.
A diver should always ascend to the surface from depth at a slow, controlled rate. If the diver has been at depth for a significant time period, he may need to make a decompression stop to allow the dissolved nitrogen in his blood to slowly and safely come out of solution. Thus, controlled buoyancy is absolutely critical when a diver surfaces. As the air in the conventional BCD bladder expands upon ascending, the diver will tend to ascend faster which, in turn, increases lift more rapidly and causes acceleration of the diver's rate of ascension, an extremely dangerous condition, especially if a decompression stop is necessary before surfacing. In order to prevent this from happening with a conventional BCD, the diver must carefully release air from the BCD during ascension. Occasionally, even experienced divers do not accurately adjust the release of air from the conventional BCD and accidentally bypass a decompression stop.
Another problem can occur when a diver, experienced or not, wears a thick and, therefore, very buoyant wetsuit. To compensate for the extra lift of the wetsuit, the diver must wear more lead in order to descend. Then, when the wetsuit compresses, the diver becomes negatively buoyant to a degree that requires a significant volume of air to be introduced into the BCD. This large volume of air will vary considerably with depth and, thus, require the diver to frequently add to and reduce the BCD volume. Similarly, if a diver is carrying heavy equipment, he will be negatively buoyant to a degree that requires significant air in the BCD, making it necessary for the diver to carefully adjust buoyancy with depth changes so as to avoid the aforementioned “uncontrolled ascent” scenario with its accompanying difficulties.
It would clearly be a desirable objective to have a BCD that provides substantially constant lift or buoyancy, or at least a degree of lift or buoyancy that varies substantially less than that of the conventional BCD, regardless of the diver's depth. Ideally, the change in volume of a BCD during ascent should produce a change in lift that is no greater than the decrease in lift that a diver can achieve with a forced expiration, which, as explained above, is on the order of 3.0 pounds. Such a BCD would result in increased safety, because the diver could then stop an ascent with a forced exhalation only.
One approach that accomplishes the objective of constant or substantially constant lift or buoyancy regardless of depth is a rigid, constant or fixed volume buoyancy tank. However, because it is often necessary to adjust buoyancy, for the reasons mentioned above, e.g., wetsuit compression and tank air depletion, most prior art fixed volume tanks provide means for adjusting buoyancy. In those prior art devices, if it is desired to adjust (increase or decrease) the lift or buoyancy of the tank, the fixed volume of the tank can be changed by a movable piston inside the tank or, more typically, by flooding the tank with water or expelling water from the tank. U.S. Pat. Nos. 3,161,028; 4,009,583; 4,068,657; 4,101,998; 4,114,389; 5,221,161; and U.S. Patent Application No. 2006/0120808 disclose buoyancy compensation devices that utilize such a rigid, constant or fixed volume buoyancy tank with means to adjust the volume. Using such devices, a diver can adjust buoyancy for a given depth to achieve vertical equilibrium and then change depth to some extent with little or no need to readjust buoyancy.
There are, however, several disadvantages to these rigid tank types of buoyancy compensation apparatus. A constant or fixed volume buoyancy tank, as the term implies, is not deflatable and thus increases the overall size (occupied volume) of the BCD regardless of depth. This adds to the cumbersome nature of the already cumbersome array of equipment a SCUBA diver needs in order to enjoy safe and interesting dives. A further disadvantage of a floodable fixed volume buoyancy compensation apparatus is that buoyancy cannot be increased or decreased when the diver is in an inverted position (or in any position in which water or air cannot be expelled to ambient through the air and water valves), unless the constant volume tank is provided with water inlet-outlet valves at both the top and bottom of the constant volume tank. Such additional valves obviously increase the cost and complexity of a fixed volume BCD, as well as make the BCD more difficult for the diver to operate. Moreover, a floodable fixed volume buoyancy tank effectively functions no differently than the conventional BCD because the pressure inside the tank corresponds to ambient pressure (as typified by the aforementioned U.S. Pat. No. 5,221,161 and U.S. Patent Application No. 2006/0120808), unless the valving of the fixed volume BCD is also designed to isolate the fixed volume of air from ambient pressure.
Referring again to the conventional BCD, that prior art device employs a collapsible or substantially flaccid, inelastic bag or bladder, the volume of which can be increased or decreased in order to control the buoyancy of the diver. The bladder is essentially inelastic during its working range of pressures because the pressures inside (internal) and outside (ambient) the bladder are the same and, when completely full or inflated to its full volume, the bladder vents air to ambient surroundings to maintain substantially equal internal and ambient pressures, as well as maintain its inelastic condition and prevent unintended rupture of the bladder. As explained above, a significant disadvantage of such a BCD is that the internal volume of the bag or bladder increases rapidly with during ascent because of the decreasing pressure of the ambient water on the bag or bladder as the diver ascends. Unless the diver offsets this rapidly increasing volume by continuously releasing air to decrease the air volume in the bag or bladder, the diver may “overshoot” his intended decompression stop depth and dangerously and rapidly ascend to the surface thereby increasing the risk of decompression sickness or pneumothorax. To minimize this problem, a number of prior art BCDs utilize automatic buoyancy control systems which include hydrostatic valving systems responsive to ambient pressure to control the volume of the bag or bladder of the BCD during the diver's ascent or descent. Obviously, the cost and complexity of such hydrostatic valving systems and the risk of failure of their numerous components are major drawbacks to this approach.
One additional disadvantage of the prior art BCD's is that the air pocket in the BCD shifts according to the diver's position. Thus, there is generally a plurality of venting valves located at different points on the BCD. Typically, a modern BCD will have three such valves. Furthermore, to fully vent a BCD, a diver must take the large corrugated tube that exits the BCD at the diver's left shoulder and extend it as high as possible above his head to vent air from the BCD. In light of these disadvantages, it would be desirable to provide a BCD that forcibly expels all the air that was introduced during the dive, so that only a single exhaust valve would be necessary and the location of that valve could be arbitrarily positioned.
Yet another disadvantage of the prior art BCD's is the shifting of the air pocket in the bladder during a dive, which can result in imbalance, causing the diver to tilt toward the right or left. It would, therefore, also be desirable to provide a BCD in which the location of the air pocket is controlled, predictable, and balanced across the diver's breadth.
The conventional BCDs in current use comprise one or more inflatable bladders in the form of a belt or vest worn on or about the torso of the diver. A few examples of such prior art BCD are disclosed in U.S. Pat. Nos. 4,913,589; 5,256,094; 5,560,738; 5,562,513; 5,707,177; 6,478,510; 6,592,298; and 6,796,744. As previously mentioned, to control the addition and expulsion of air from the bladder, many prior art BCDs include mechanisms for automatically adjusting the buoyancy of the BCD bladder depending on the water depth or pressure or other parameters. Several of such prior art automatic BCDs are disclosed in U.S. Pat. Nos. 3,820,348; 5,496,136; 5,560,738; and 6,666,623. U.S. Pat. No. 5,551,800 discloses an automatically adjustable BCD which uses an expandable bellows with an internal spring as a buoyancy bladder or tank.
In light of the foregoing, it would be desirable to provide a BCD for a diver which has a relatively constant buoyancy regardless of diving depth, i.e., approaching that of a rigid, or fixed volume BCD, without the disadvantages of having a more complex and cumbersome buoyancy compensation system. It would also be desirable to provide a BCD with a displacement container or volume having a change of buoyancy with depth (lift versus depth characteristic) that approaches the constant lift versus depth characteristic of a rigid tank BCD and has a lift versus depth characteristic substantially better than that of the conventional BCD that employs an ambient pressure inflatable inelastic bladder for buoyancy control. Furthermore, it would be desirable to provide a BCD that overcomes the foregoing limitations and shortcomings of prior art BCD devices, has a minimum number of parts, is economical to manufacture and is easy, convenient and particularly safe for a diver to use.